^^Sounds like my son and his friends (16/17). They'll say they use metric, but if you listen, they use a mixture. Because "it's sense, everyone does that" or something similar.
The measurements are all arbitrary, so they all have relevance and weight. Except length, which has no weight. Just relevance. 
A mixture is good. We want our young people to grow up being able to cope with different and difficult situations. Don't we...?
Or do you want them to go off to foreign lands, shouting (slowly) that xxxx measurement is best? You want them to grow up understanding that people are different and live in different ways; and that if the differences are merely about some arbitrary physical measurement, that there is a way to deal with that.
No longer do people rely on log tables and long division. That's what computers are for, and they can work out conversions to far more decimal places that any human can. It's what IT is for.
Who the hell made 10 the best number anyway? Blake Edwards?
I had to look up Blake Edwards. Found he wasn't the leader of Blake's Seven.
Yes, all of that. But more that usage is shaped by environment. If they were growing up in Germany or USA, they'd probably be mono-systemic, or whatever the term should be.
Obviously this means that
BRITISH brains, forced to adapt to an ever-changing mix of measures, are far more flexible than the rigid brains of the rest of the world. Except maybe for Ireland, Canada, Australia and even the USA. And we're all less flexibly-brained than people living in the Mediterranean during the Roman Empire, who had to deal with totally different time and date systems from place to place.
